Heat exchangers are known that are used, for example, in the petrochemicals industry or oil refining, which are made in the form of plate heat exchangers and which serve to exchange heat with very good efficiency between a hot fluid and a cold fluid, with the cold fluid being subjected to a temperature rise of the order of 300° C. to 400° C., for example.
In such applications, plate heat exchangers have the advantage of presenting a very good coefficient of heat exchange.
In general, such heat exchangers comprise one or more bundles of plates each constituted by a stack of superposed plates that extend parallel to one another and that define between them two totally separate flow circuits for two fluids.
Each of the individual plates in a plate heat exchanger bundle is constituted by a fine metal sheet, e.g. made of stainless steel, that is shaped so as to have corrugations of a special shape in a central zone of the plate, through which heat is transferred between the fluids.
The corrugations of the plates in the heat exchanger bundle are disposed so as to be adjacent to one another and cover the entire surface area of the central zone of the heat exchanger plate. The corrugations may be directed in a longitudinal direction of the plate which constitutes a general flow direction for the fluids between which heat is exchanged.
By way of example, the fluids may be caused to flow as counterflows, i.e. in directions that are parallel but in senses that are opposite on opposite sides of the plates stacked one above another in the heat exchanger bundle.
The corrugations of each plate directed along a longitudinal axis of the plate between an inlet end portion and an outlet end portion of the plate, themselves comprise substantially rectilinear segments following one another in the longitudinal direction and extending obliquely relative to said plate direction. The successive and oblique segments are inclined relative to the longitudinal axis of the plate along which they are disposed successively to one side and to the other side of the longitudinal axis so as to constitute a zigzag line. The adjacent corrugations constitute ridge lines in a first face and also in an opposite second face of the heat exchanger plate.
The plates of a plate heat exchanger bundle that are stacked one on another are disposed in alternation in first and second dispositions, the stacked plates being turned through 180° face for face relative to the two adjacent plates in the stack. Thus, the plates referred to in alternation as being odd plates and as being even plates present corrugations whose superposed rectilinear segments have orientations that are different. As a result, the plates rest on one another via their corrugations touching in zones of substantially point contact.
The plates stacked on one another in a bundle are generally of rectangular shape and are connected to one another along their longitudinal edges by connection means that provide leaktight closure of the lateral sides of the bundle. Plane sheets disposed at the top and bottom of the stack and fixed to the lateral connection means also serve to close the top and bottom portions of the bundle of plates.
The successive rectilinear segments of the longitudinal corrugations of the heat exchanger plates make obtuse angles relative to one another that are very wide open, each of the successive segments being inclined little relative to the longitudinal axis relative to which it is obliquely disposed.
This disposition of successive segments of the corrugations puts a considerable limit on the ability of the plate to lengthen inside the heat exchanger while it is in operation and under the effect of expansion due to making contact with a fluid at high temperature. The plates are very rigid in the longitudinal direction because of the small inclination between the successive segments of the corrugations.
Stresses of thermal or mechanical origin to which the heat exchanger is subjected therefore need to be absorbed by each of the plates in the plate heat exchanger and also by the entire bundle of plates in the assembled state.
This can lead to excessive stresses in the plates which are fine metal sheets and also in the structures of the bundle and of the heat exchanger.